In Situ and Operando X-Ray Absorption Spectroscopy On Cu-Zeolite Catalysts for Fast SCR by Ammonia

NO_x removal by selective catalytic reduction (SCR) has been studied extensively by the exhaust aftertreatment community.  Cu-zeolites, especially Cu on ZSM-5, have been found to be particularly well suited for low temperature SCR [1].  Despite a large body of research on Cu-ZSM-5, the debate about the active Cu species is still ongoing.  Our recent measurements of the Cu X-ray Absorption Near Edge Spectra (XANES) in a custom-built, operando reactor using glassy carbon tubes have produced intriguing results.  In situ XANES spectra were collected over a range of Cu-ZSM-5 catalysts, from loadings of 1-7 wt.% Cu, under NO oxidation, dry SCR, wet SCR and pure ammonia gas conditions and a 2.7 wt.% Cu was tested operando under wet and dry NO oxidation and wet SCR reaction conditions.  During NO oxidation, the Cu was split between isolated Cu²⁺ species and Cu²⁺ in the form of bulk-like CuO, present in small amounts at lower loadings and becoming the dominant species at high loadings where the Cu over-exchange was around 300%. Nevertheless, the Cu was fully oxidized.  Upon the introduction of the reducing agent, ammonia, for both dry and wet SCR conditions, a significant portion of Cu reduced to its 1+ oxidation state.  Similar to the Cu²⁺, however, this 1+ species was an isolated Cu¹⁺, entirely different from the Cu¹⁺ found in bulk Cu₂O.  In fact, there was never any identifiable amount of Cu¹⁺ in a bulk-like Cu₂O form during any phase of our experiments.  Lower Cu wt.% loadings (50-100% exchanged) had higher proportions of reduced Cu where the lowest Cu loading showed the presence of very little Cu²⁺.  The amount of reduced Cu¹⁺ ranged from 5-70% going from high to low Cu loadings.  Furthermore, at low weight loadings, some Cu²⁺ was identified as Cu coordinated with NH₃ where the higher wt.% Cu samples appeared to be coordinated with H₂O instead.  Implications of the effects of the presence of reduced states of Cu on SCR and NO oxidation kinetics will be discussed. 

[1] S. Brandenberger, O. Kröcher, A. Tissler, R. Althoff.  Catalysis Reviews, 50:4, 492-531, 2008